Almost immediately after the Supreme Court ruled that human genes could not be patented, several laboratories announced they, too, would begin offering genetic testing for breast cancer risk, making it likely that that test and others could become more affordable and more widely available.

The ruling in effect ends a nearly two-decade monopoly by Myriad Genetics, the company at the center of the case.

“It levels the playing field; we can all go out and compete,” said Sherri Bale, managing director of GeneDx, a testing company, which plans to offer a test for breast cancer risk. “This is going to make a lot more genetic tests available, especially for rare diseases.”